🤝 add CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing to argminmax
+
+Welcome! We're happy to have you here. Thank you in advance for your contribution to argminmax.
+
+## The basics
+
+argminmax welcomes contributions in the form of Pull Requests. For small changes (e.g., bug fixes), feel free to submit a PR. For larger changes (e.g., new functionality, major refactoring), consider submitting an [Issue](https://github.com/jvdd/argminmax/issues) outlining your proposed change.
+
+### Prerequisites
+
+argminmax is written in Rust. You'll need to install the [Rust toolchain](https://www.rust-lang.org/tools/install) for development.  
+
+This project uses the nightly version of Rust. You can install it with:
+
+```bash
+rustup install nightly
+```
+
+and then set it as the default toolchain with:
+
+```bash
+rustup default nightly
+```
+
+### argminmax 
+
+The structure of the argminmax project is as follows:
+
+```bash
+argminmax
+├── Cargo.toml
+├── README.md
+├── src
+│   ├── lib.rs     # ArgMinMax trait implementation
+│   ├── scalar     # Scalar implementation
+│   ├── simd       # SIMD implementation
+├── benches        # Benchmarks
+├── tests          # Integration tests
+├── dev-utils      # Helper functions (for testing & benchmarking)
+```
+
+The Rust code is located in the `src` directory. The `lib.rs` file contains the trait implementation. The `scalar` and `simd` directories contain the scalar and SIMD implementations respectively.
+
+The `simd` directory contains a `generic.rs` file that contains the `SIMD` trait - can be seen as an interface that all the SIMD instruction sets x data types must implement. The implementations of this `SIMD` trait are located in the `simd_xxx.rs` files (e.g., `simd_f32.rs`).
+
+### Testing
+
+Unit tests are located at the bottom of (almost) every Rust file. The integration tests are located in the `tests` directory.
+
+They can be run with:
+```bash
+cargo test --all-features
+```
+
+### Benchmarking
+
+The benchmarks are located in the `benches` directory - they are written using [criterion](https://docs.rs/criterion/latest/criterion).
+
+To run the benchmarks, use the following command:
+
+```bash
+cargo bench --quiet --message-format=short --features half | grep "time:"
+```
+
+### Formatting 
+
+To format the Rust code, run the following command:
+```sh
+cargo fmt
+```
+
+---
+
+## Improving the performance
+
+When a PR is submitted that improves the performance of the library, we would highly appreciate if the PR also includes a (verifiable) benchmark that shows the improvement.